What condition would classify soil as unstable?

Soil is classified as unstable when it shows fissures or is subject to vibration because these conditions indicate a lack of cohesion and structural integrity within the soil. Fissures can signify that the soil is experiencing stress or movement, suggesting it cannot adequately support loads applied to it. Similarly, vibration, whether from nearby construction activities or heavy machinery operation, can disturb the soil's particles, further destabilizing it. This instability can lead to potential hazards such as landslides or subsidence, making it crucial for excavator operators to recognize and assess these signs before commencing work.

In contrast, the other choices represent conditions that are typically associated with stable or manageable soil conditions. High unconfined compressive strength generally indicates that the soil can bear weight effectively. Balanced moisture content does not suggest instability, as it typically contributes to optimal soil performance. A dry state, while sometimes a concern depending on other factors, does not inherently classify soil as unstable. Thus, recognizing the signs of fissures and the effects of vibration is key to ensuring safety and structural integrity when working with soil in excavation contexts.